But then the described service is useless, because you can only trust the confidentiality of your mail in so far as you can trust the honesty of the service pro

But then the described service is useless, because you can only trust the confidentiality of your mail in so far as you can trust the honesty of the service provider not to hide backdoors in the js they send to your browser. But then you can just do away with the clunky client side cryptograpy and talk plain https with their server that does the encryption.